This invention relates to automatic clothes dryers and more specifically to control systems therefor designed to terminate the clothes drying cycle when the percentage of moisture retained in the clothes load has declined to a level of dryness desired by the user.
In present day commercially available automatic clothes dryers, it is common practice to provide an electromechanical timer by which the user may set the drying time to a value that the user expects will result in a desired degree of dryness for the particular clothes load involved. Although the timer dial is generally provided with markings intended to assist the user in selecting the proper amount of dryer time, the user is generally left with a need to estimate, based on experience, the amount of time needed to dry a particular load. It is known, however, that the required amount of drying time is dependent on such factors as the particular design of the dryer, the amount of heat acquired via the dryer air flowing through the dryer, the size of the clothes load, and the moisture content of the clothes load at the beginning of the drying cycle. U.S. Pat. No. 4,019,259 teaches that for a given temperature of inlet air to the dryer, a family of time temperature characteristic curves may be derived, each of which exhibits a substantially constant temperature plateau at a temperature value which is a function of the clothes load size. Thus a small clothes load exhibits a constant temperature plateau which is higher than a larger clothes load. The patent goes on to disclose an electromechanical control system for terminating clothes dryer operation based on the amount of time calculated electromechanically by the cooling effect of the clothes load. In effet, the duty cycle of a thermostatic switch is varied for a shorter dryer time with small loads than with larger loads.
Electromechanical arrangements of the type involved in the foregoing device are subject to long term reliability problems associated with repeated operation. Accuracy of control can also be a problem dependent upon the closeness with which the thermostatic switches can be manufactured to design specifications. Moreover, the approach suggested by the U.S. Pat. No. 4,019,259 assumes that a given drying time for a given clothes load dryer air temperature plateau will result in a desired degree of dryness of the clothes load. This is a predictive form of control which, although not necessarily entirely unsatisfactorily, does not assure that a particular dryness level is in fact reached when the dryer cycle is terminated.
It is, therefore, an object of the present invention to provide a control system for an automatic clothes dryer which obviates the problems associated with electromechanical controls.
It is a further object of the present invention to provide a control system for an automatic clothes dryer which provides a more positive form of dryer termination control than heretofore available with electromechanical control arrangements.